1. Field of the Invention
This invention relates to a reusable electrophotographic photosensitive member, and more particularly to a reusable electrophotographic photosensitive member which was a coherent light as an incident light during image formation.
2. Description of Related Art
Electrophotographic photosensitive members utilizing inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc. as photosensitive components have been so far well known.
On the other hand, since it was found that specific organic compounds have a photoconductivity, so many organic photoconductive materials have been so far developed. Known are, for example, organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, etc.; low molecular weight organic photoconductive materials such as carbazoles, anthracenes, pyrazolines, oxazoles, hydrazones, polyarylalkanes, etc.; and organic pigments or dyes such as phthalocyanine pigments, azo pigments, cyanine pigments, polycyclic quinone pigments, perylene-based pigments, indigo dyes, thioindigo dyes, squarilium dyes, etc.
Moreover photoconductive organic pigments or dyes can be more readily synthesized than the inorganic materials, and have a broader range of variations available for selecting compounds having a photoconductivity to an appropriate wavelength region, and so many photoconductive organic pigments and dyes have been so far proposed. Known are, for example, electrophotographic photosensitive members using a disazo pigment exhibiting a photoconductivity as a charge generating material in a photosensitive layer which is functionally separated into a charge generation layer and a charge transport layer, etc., as disclosed in U.S. Pat. Nos. 4,123,270; 4,247,614; 4,251,613; 4,251,614; 4,256,821; 4,260,672; 4,268,596; 4,278,747; and 4,293,628.
Generally, such electrophotographic photosensitive members as above have been developed mainly to obtain a higher sensitivity, less dependence upon environmental conditions, and constantly maintained characteristics. A reusable type, that is, an electrophotographic photosensitive member can be repeatedly used in the formation of images by removing the remaining developing agent therefrom after the formation of an image, has been so far used, owing to its simplicity, in the system for forming an image by means of the electrophotographic photosensitive member.
In the electrophotographic photosensitive member based on such a type, compatibility with a means for removing a developing agent is an important characteristic besides the other various characteristics of electrophotographic photosensitive member. If the compatibility with the means for removing the developing agent is poor, the surface of the photosensitive member will be damaged when the developing agent is fixed to or removed from the surface of the photosensitive member, restricting the repetition which may be run to a smaller number. Furthermore, the surface resistance will be lowered by deposition and tracking of materials of low resistance produced due to the surface deterioration of photosensitive member or due to the electrocharging process, resulting in the image unfocusing. To practically satisfy all of these requirements, so many attempts have been so far made, for example, by improvements of the developing agent, improvements of the means for removing the developing agent, improvements of processes to be used, improvements of physical properties of the photosensitive member, inclusion of a lubricant, etc.
However, these attempts are dependent entirely upon a combination of several kinds of techniques. Thus, it has been very difficult to obtain their effects at the same time or cost increase, etc. have been inevitable.
Particularly, an organic photosensitive member has weak mechanical strength and, when applied for a copying machine, a printer, etc., will suffer from formation of pinholes, minute cracks, abrasion at the end portions, peel-off, etc., to result in image defects.
For this reason, for increasing the mechanical strength of the organic photosensitive member, photosensitive members having particles dispersed in the photosensitive layer have been investigated. However, since particles cannot be dispersed uniformly, whereby pinholes, cracking, peel-off, etc. in photosensitive member are worsened to cause increased image defects.
In the image-forming process, selenium, selenium-based alloys, cadmium sulfide resin-distributed systems, charge transfer complexes of polyvinylcarbazole and trinitrofluorenone, etc. have been used as photosensitive materials for the electrophotographic printer using a coherent light, typified by laser, as a light source. As a laser, gas lasers such as helium-cadmium laser, argon laser, heliumneon laser, etc. have been used, and a semiconductor laser of small size and low cost, capable of direct modulation, has recently been available. However, most of the semiconductor lasers have an emitted light wavelength of 750 nm or more, and the photosensitive materials have a low photosensitivity in such a wavelength region, and have not been widely utilized. Under these circumstances, a lamination type photosensitive material composed of a charge generation layer and a charge transport layer has been regarded as an important photosensitive material for the semiconductor laser printer, because the photosensitive wavelength range can be relatively freely selected.
The charge generation layer of the lamination type photosensitive material plays a role of absorbing light to generate free charges, and to make the range of generated photocarriers shorter its thickness is usually as small as 0.1 to 5 .mu.m. This is ascribable to absorption of most of the incident light in the charge generation layer, forming many photo carriers and the necessity for injecting the generated photo carriers into the charge transport layer without any deactivation due to recombination or trapping.
The charge transport layer plays a role of receiving static charges and transporting free charges without any substantial absorption of image-forming light, and its thickness is usually 5-30 .mu.m. When images are produced by line scanning of a laser beam in a laser printer, using such a lamination type photosensitive material as above, line images such as letters, etc. have no problem, but black tone images have an uneven image density in an interference fringe state.
The cause for the development of the interference fringes seems to be that the charge generation layer is formed as a thin layer, as described above, and thus the quantity of light absorbed in the charge generation layer is so restricted that the light passed through the charge generation layer is reflected on the surface of the electroconductive support, causing the reflected light to undergo interfere with light reflected on the surface of the photoconductive layer.
The conventional lamination type electrophotoconductive photosensitive member comprises a charge generation layer 4 on an electroconductive support 3 laid on a support 2, and a charge transport layer 5 laid on the charge generation layer 4. When an incident layer beam 6, whose oscillation wavelength is about 780 nm in the case of a semiconductor laser and about 630 nm in the case of a helium-neon laser, is allowed to enter into the said lamination type electrophotographic photosensitive member, an interference develops between the incident light 7 to the charge transport layer and further to the inside of the photosensitive layer, and another reflected light 9 obtained by reflection of the incident light 7 on the electroconductive support 1 and emitted from the surface of the charge transport layer 5.
Let the refractive index of the lamination comprising the charge generation layer and the charge transport layer n, its thickness d, and the wavelength of laser beam .lambda.. When nd is an integral multiple of .lambda./2, the intensity of the reflected light becomes a maximum, that is, the intensity of the light entering into the charge transport layer becomes a minimum according to the principle of the conservation of energy, whereas, when nd is an odd multiple of .lambda./4, the intensity of the reflected light becomes a minimum, that is, the intensity of the light entering into the charge transport layer becomes a maximum. However, the thickness d inevitably has an unevenness in the order of at least 0.2 .mu.m inherent to the available production technique.
On the other hand, it is preferable that the laser beam is monochromatic, but the laser beam is coherent and thus the said interference conditions change in accordance with the unevenness in the thickness. That is, it seems that the quantity of a laser beam absorbed in the charge generation layer becomes locally uneven, causing to develop an uneven area image density in an interference fringe state.
In the ordinary electrophotographic copying machines, no monochromatic light is used as the light source, and thus the width of an interference fringes as a cause for the uneven density changes with the wavelength, and the uneven density disappears by the consequent balancing.
In the electrophotographic process using a laser beam, the development of uneven density in the interference fringe state has been so far prevented, for example, by roughening the reflecting surface of the support or the lamination interface for the electroconductive layer or the photosensitive layer, thereby providing an unevenness thereon to give a phase difference to the reflected light. However, in the case of the lamination type electrophotographic photosensitive member, a uniform photosensitive layer cannot be formed on such a uneven surface as obtained by the surface roughening, resulting in an image defect or considerable deterioration of photographic characteristics.
On the other hand, the method utilizing a photosensitive layer surface layer has been also investigated. That is, the techniques of effecting diffused reflection by such method as addition of coarse irregular shaped particles, irregular shaped fine particles with great agglomerating tendency, etc. have been known. However, none of them can control dispersion of the particles, to cause image defects as mentioned above.
Addition of coarse irregular shaped particles or irregular shaped particles with great agglomerating tendency, which may be techniques effective for diffused reflection, are susceptible to formation of irregular coarse defects on the photosensitive layer surface, thus giving rise to great problems such as black dots, fog, etc. on the image under the present situation.
Practically, although addition of coarse irregular shaped particles with an average particle size of 2 .mu.m or more can cause diffused diffusion to occur effectively within the charge transport layer, such particles are generally liable to be sedimented and can maintain uniform state in a coating solution formulated by dispersion with difficulty and therefore it is difficult to produce them stably under the present situation.
On the other hand, fine particles with irregular shapes of 0.5 .mu. or less, have generally no effect of causing diffused reflection to occur within the charge transport layer, when dispersed uniformly in a binder solution. However, in the case of irregular shaped large particles with great agglomerating tendency or relatively poor affinity between the particles and the binder, it is possible to effect diffused reflection within the charge generation layer by agglomeration of fine particles. However, in this case, the degree of agglomeration can be controlled with extreme difficulty, whereby not only irregular and great defects are formed on the surface, but also agglomeration of fine particles occurs in the coating solution, and therefore it is very difficult to obtain production stability, as required for practical application.
As is obvious from the foregoing, an electrophotographic photosensitive member must have a specific sensitivity, electrical characteristics and optical characteristics applicable to an electrophotographic process. Particularly in the case of repeatedly usable electrophotographic photosensitive member, durability is further required against electrical and mechanical external forces such as corona charging, toner development, transfer to paper, cleaning treatment, etc. as applied directly to the surface layer of the electrophotographic photosensitive member. Specifically, durability is required against a decrease in the sensitivity or potential or an increase in the residual potential, caused by deteriorations due to the ozone generated during the corona charging, and also against attrition or damage on the surface due to the sliding friction.
On the other hand, the moisture resistance of the electrophotographic photosensitive member is also another important property. If the surface potential of an electrophotographic photosensitive member is considerably lowered at a high humidity, it is difficult to obtain a stably clear image, even though the electrophotographic photosensitive member has distinguished electrophotographic characteristics at a low humidity. Furthermore, in a transfer-type electrophotographic process, the electrophotographic photosensitive member is usually used repeatedly, and the moisture resistance is more liable to decrease owing to the electrocharging deterioration of the electrophotographic photosensitive member
The decrease in the moisture resistance can be overcome to some degree by heating the electrophotographic photosensitive member, thereby drying it. However, the heater must be always operated, resulting in a cost increase.
Furthermore, deposition of paper dust through contact with paper is a cause for smeared image at a high moisture, and residual toner due to the toner filming or poor cleaning considerably deteriorates the resulting image.